The ongoing conversion of indoor lighting to energy efficient LED systems offers enormous opportunity for increasing the functionality of lighting from today's modest on/off/dimming control to a new Smart Lighting paradigm that takes advantage of LEDs' electronic compatibility and flexibility. This new lighting paradigm includes lighting for enhanced worker/student productivity, health effects such as circadian entrainment reinforcing the human sleep/wake cycle, visible light communications (VLC) to alleviate the growing wireless bottleneck, and occupancy/activity sensing to provide custom lighting.
The highest lighting efficacy will be achieved with multiple LEDs at different colors across the visible, eliminating the energy losses inherent in phosphor color conversion. VLC will require a multiple-input multiple-output (MIMO) architecture with multiple LEDs from multiple fixtures to provide the necessary aggregate Gbps data rates and to support mobility as people move with their personal devices. Light has many impacts on health and productivity; spectral as well as intensity variations are important for optimizing the human environment. An even greater energy savings, along with a more comfortable experience, is available by adapting lighting to human activity in addition to the savings from the improved efficacy of LEDs.
Currently there is a trend to develop smart lighting that involves multiple LEDs in each light fixture with 4 to 10 independent colors spanning the 400-nm to 700-nm visible spectrum. There is a need in the art to provide smart lighting that will allow a broad color gamut, but also require a sophisticated control system to adapt to different lighting conditions, different impacts of furnishings and walls, floors, ceilings, and/or different aging of the LEDs in different fixtures. While today's color cameras include components, such as photosensitive pixels that could be integrated for use in smart lighting, the angular and spectral resolution requirements needed for smart lighting sensors are quite different than those of traditional cameras that require angular insensitivity and only have pixels with three relatively broad-band and spectrally overlapped color filters (RGB). Most commonly, today's color cameras utilize dye absorbers, typically with spectral bandwidths of ˜100 nm or greater that are located directly atop the silicon photosensitive pixels of the camera.
Attempts at developing technology for smart lighting components include focal plane color filters for application to the color pixels of digital cameras. Surface plasma wave (SPW) enhancement of semiconductor detectors has been extensively investigated in the infrared spectral region. Typically, in the IR the approach is to couple to a SPW bound to the metal-semiconductor interface. This allows the use of a thinner absorption region (with, therefore, lower noise currents) and a longer absorption path (along the pixel rather than across the junction depth). However, this approach is not appropriate for the visible spectrum due to the high, and strongly varying, absorption of silicon across the visible spectrum. Another issue is the small scale of the required grating which is ˜λ/n with n, the semiconductor refractive index, of 4 to 5 for silicon across the visible spectrum. Additionally, limitations of the SPW approach include: 1) the relatively high metal optical losses in the visible restrict the available bandwidths; spectral widths are typically 100 to 200 nm, an order of magnitude larger than the desired bandwidths; and 2) the transmission is low, typically no larger than 10%, limiting the sensitivity of the measurement.
While there have been many demonstrations of far-field filtering (plane wave to plane wave), relatively few demonstrations of coupling to silicon materials for detection have been presented. In such few demonstrations, linewidths have been broad, typically 100 to 200 nm. The term “plasmonics” generally covers both extended (propagating) surface plasma waves (SPW) defined on a metal-dielectric interface and localized surface plasma resonances (SPR) associated with metal particles, holes in a metal film, discs of metal, etc. The angular responses SPW's and SPR's are quite different with SPW's having a narrow angular response depending on the periodicity of the surface while SPR's have generally angularly independent responses. In any real plasmonic structure these two resonances interact giving a complex, wavelength dependent angular response. Meanwhile, pixels are generally small, driven by trends in high-pixel count cameras where individual pixels are sub-10 microns. Further, many studies have demonstrated a far-field filter approach based on extraordinary optical transmission through arrays of holes in a metal film where the far-field transmission of the filter is used as the spectrally selective quantity. That approach is difficult to achieve in a convenient form factor as a result of the long propagation distances required to achieve a far field regime, requiring standoff of the filter element from the silicon detector array.
Other work has focused on radiation coupling with a 2D waveguide fabricated on a substrate. For example, guided-mode resonance (GMR) filters, consisting of a grating coupler and a single mode slab waveguide on a transparent substrate have demonstrated both angular and spectral sensitivity in reflection and transmission. Off-resonance, GMR filters simply act as a dielectric medium, usually with the majority of the incident power simply being transmitted. On resonance, the grating couples some of incident photons into the waveguide and the propagating photons in the waveguide are coupled back into the reflected and transmitted beams. As a result of the phase shifts inherent in this process, the out-coupled photons reinforce the reflected wave and interfere destructively with the directly transmitted light to reduce the transmitted power. Since the waveguide is lossless and the grating is large (many wavelengths), an extremely narrow resonance response is achieved.
Waveguide integrated optics at telecommunications wavelengths has demonstrated that grating coupling into waveguide modes can provide the necessary spectral and angular filtering with recent demonstrations of only 0.6 dB loss in conversion from a 2D waveguide to a single mode fiber.
What is needed in the art is a device that comprises color pixels with both color and angular sensitivity that can be integrated onto a silicon surface with a scalable, manufacturable process (e.g., not requiring separate fabrication steps for each desired wavelength/angle setting), providing both manufacturing convenience and reduced form factors.
Further, the silicon absorption varies considerably across the visible. At blue wavelengths (about 400 nm) the absorption of silicon is quite strong with a 1/e absorption length of only ˜100 nm. In contrast at the red end of the spectrum (about 700 nm) the silicon 1/e absorption length is ˜8 micrometers (80× longer). As a consequence, the responsivity of silicon photodetectors also varies across the visible. For blue sensitivity, the junction depth must be quite shallow, within the short 1/e absorption length, which is difficult to accomplish with traditional CMOS fabrication processes. Therefore, another aim of the invention is to provide a CMOS compatible p-n junction technology that accommodates the short penetration depth of blue photons into silicon.